Replace soon-to-be-deprecated exo-string functions

Related : exo#82 (closed)

Requires Glib >= 2.66.0, libxfce4util>=4.17.2

• _exo_string_looks_like_an_uri () => g_uri_is_valid ()

A code is duplicated from libexo (probably to avoid dependency), but a function that can do such is added in Glib 2.66.

• exo_str_is_empty () => xfce_str_is_empty ()

This function is relocated from libexo to libxfce4util.

Note: Dependency update is separated to explicitly show what dependency is changed in commit history.